The present invention relates to a gray-scale image printing method used for a thermal printer which uses a thermal head having a plurality of printing elements (or heating elements) arranged thereon, and more particularly, to an image printing method which utilizes a dither pattern.
There has been proposed an image printing method that involves producing a gray-scale image by changing a period of time during which power is applied to heating elements of a thermal printer so as to change the area of printing dots, each of which is printed on printing paper as a single dot.
Under this method, an image is printed on printing paper while a thermal head is actuated in a secondary scanning direction (i.e., in a direction perpendicular to the direction in which the heating elements are arranged) for each color component. Accordingly, color pixels may sometimes deviate from one another in the secondary scanning direction. In the event of the pixels"" deviating from one another (hereinafter referred to as xe2x80x9cmisregistrationxe2x80x9d), moire (interference fringes) appears, resulting in a change in color tone and an image differing in color from the original image.
As a technique of eliminating a change in color tone stemming from such moire or misregistration, there is described an image printing method in Hei. 7-312677.
This image printing method (i.e., a VR screen method) involves forming a color dot pattern for each color, which has desired resolution in the secondary scanning direction, by changing dot pitch in the same direction at a desired magnification and by superimposing the thus-prepared different color dot patterns one over another so as to create an image in multiple colors. Consequently, chromatic misregistration is prevented from arising in one particular direction, thus preventing chromatic moire.
In the existing heat-transfer thermal printer, ink applied over an ink ribbon is heated by heating elements, and the thus-heated ink is transferred to printing paper. Such a configuration makes it difficult to change the tone of a pixel comprising one dot. To solve the problem, there has been proposed a method using a dither pattern which enables a heat-transfer system to produce a gradient expression.
Under this method, an Mxc3x97N array of dots is handled as one block or one unit matrix, and the combination of the number of dots to be transferred within one block and the dot size is controlled so as to enable a gradient expression.
In a case where the VR screen method used for the purpose of preventing chromatic misregistration and the dither pattern enabling a gradient expression are used in combination, an actual dot pitch is determined without reference to resolution by means of the dither pattern, posing a problem of the VR screen method, which prevents chromatic misregistration by changing the dot pitch for each color, having difficulty in sufficiently preventing chromatic misregistration.
The present invention has been conceived in light of the foregoing problems, and the object of the present invention is to provide an image printing method capable of preventing chromatic misregistration even when dither patterns are utilized.
The present invention provides an image printing method, wherein one pixel is formed from a plurality of dots arranged in the primary scanning direction, in which printing elements are arranged, and in the secondary scanning direction perpendicular to the primary scanning direction; wherein the tone of one pixel is expressed by changing the overall printing area in accordance with whether or not the dots are printed; and wherein a multicolor image is printed by combination of one yellow pixel, one magenta pixel, one cyan pixel, and one black pixel, the method comprising the step of: setting to a nonintegral multiple a dimensional ratio among one yellow pixel, one magenta pixel, one cyan pixel, and one black pixel in the secondary scanning direction.
In the image printing method, the dimensional ratio among one yellow pixel, one magenta pixel, one cyan pixel, and one black pixel in the secondary scanning direction is preferably set to one selected from the group comprising, in no particular order, (10:12:15:20), (8:10:12:15), (15:20:24:30), (21:24:28:42), (30:35:42), (10:12:15), (8:9:12), (2:3:4), (5:6:10), (5:8:10), (7:8:14), and (3:4:6).
Furthermore, the present invention also provides an image printing method, wherein one pixel is formed from a plurality of dots arranged in the primary scanning direction, in which printing elements are arranged, and in the secondary scanning direction perpendicular to the primary scanning direction; wherein the tone of one pixel is expressed by changing the overall printing area in accordance with whether or not the dots are printed; and wherein a multicolor image is printed by combination of one yellow pixel, one magenta pixel, one cyan pixel, and one black pixel, the method comprising the step of: setting to a nonintegral multiple a dimensional ratio among one yellow pixel, one magenta pixel, one cyan pixel, and one black pixel in the primary scanning direction.
In the image printing method, the dimensional ratio among one yellow pixel, one magenta pixel, one cyan pixel, and one black pixel in the primary scanning direction is preferably set to one selected from the group comprising, in no particular order, (10:15:20).
According to the foregoing image printing method, since the size ratio among the single color pixels in the secondary scanning direction is set to a nonintegral multiple, the pixel pitch of each color is changed, thus preventing chromatic misregistration in the same manner as in the case of a VR screen method, which prevents chromatic misregistration by changing dot pitch. Further, it is also possible to set the size ratio among the single color pixels in the primary scanning direction to a nonintegral multiple. Even in such a case, chromatic misregistration can be prevented in the manner analogous to that described previously.
The dimension or size of one pixel in the primary or secondary scanning direction used herein signifies the length of a unit matrix in the primary or secondary scanning direction.
Further, a nonintegral multiple used herein signifies that the dimensional ratio of an arbitrary color pixel to at least one another color pixel in the secondary scanning direction is a nonintegral multiple. In short, in the case of a dimensional ratio among single color pixels being (10:12:15:20), if 10 is selected as the size of an arbitrary color pixel, a ratio of 12 to 10 can be represented as a nonintegral multiple of 1.2. In the case of a ratio of 15 to 10, there is obtained a nonintegral multiple of 15. However, in the case of a ratio of 20 to 10, there is obtained an integral multiple of 2. Of the three ratios, two represent nonintegral multiples. Further, even when 12 is selected as the size of an arbitrary color pixel, all the three ratios represent nonintegral multiples. Still further, even when 15 is selected as the size of an arbitrary color pixel, all the three ratios represent nonintegral multiples. Yet further, when 20 is selected as the size of an arbitrary color pixel, two of the three ratios represent nonintegral multiples. Accordingly, in the case of the foregoing example, since at least a dimensional ratio of an arbitrary color pixel to at least one another color pixel in the secondary scanning direction represents a nonintegral multiple, the dimensional ratio (10:12:15:20) can be said to be a ratio of nonintegral multiple.